Hundreds of thousands of Americans in poorer counties have few choices of health insurers and face high premiums through the online exchanges created by the health-care law, according to an analysis by The Wall Street Journal of offerings in 36 states.

Consumers in 515 counties, spread across 15 states, have only one insurer selling coverage through the online marketplaces, the Journal found. In more than 80% of those counties, the sole insurer is a local Blue Cross & Blue Shield plan. Residents of wealthier, more...